Erythema nodosum: causes, diagnosis, and treatment

Erythema nodosum is the most common form of panniculitis, an inflammation of the fat layer beneath the skin. It appears as tender, red to purple nodules, usually on the front of both shins, that feel like firm, warm lumps and can make walking uncomfortable [1][2]. It is less a disease in itself than a reaction: the skin is responding to a trigger elsewhere in the body, which is identified in about half of cases and remains unknown in the rest [1][2]. In most people the nodules settle within a few weeks without scarring.

This article explains what erythema nodosum is, the conditions that commonly trigger it, how it is diagnosed, and how it is treated. It is not medical advice; diagnosis and treatment should be directed by a physician, because erythema nodosum can be the first visible sign of a systemic illness.

What erythema nodosum is

Erythema nodosum appears as a sudden crop of painful, erythematous nodules under the skin, most often on the pretibial area, the front of the lower legs [2]. The lesions are typically bilateral and symmetrical and range from about 1 to 5 cm across [2]. As they heal they often flatten and change color like a fading bruise, without leaving an ulcer or scar. Because it reflects a hypersensitivity response to a range of possible triggers rather than a single cause, the same appearance can arise from very different underlying processes [1].

The condition is idiopathic, meaning no cause is found, in roughly half of cases [1][2]. When a cause is identified, treating or resolving it is what ultimately settles the skin reaction [1].

Common causes and triggers

Erythema nodosum can accompany a wide range of conditions [2]. The more commonly identified triggers include [1]:

• Infections, especially streptococcal throat infection, and in some regions primary tuberculosis [1][2].
• Sarcoidosis, an inflammatory condition that can involve the lungs and lymph nodes [1].
• Inflammatory bowel disease, including Crohn disease and ulcerative colitis [1][3].
• Medications, such as certain antibiotics and oral contraceptives [1][2].
• Pregnancy and some autoimmune conditions [1][2].

Less commonly, it can be linked to vaccination or, rarely, an underlying malignancy [2]. In inflammatory bowel disease, erythema nodosum is one of the more common skin-related extraintestinal manifestations, and it often tracks with intestinal inflammation, tending to improve when the bowel disease is brought under control [3][4]. Extraintestinal manifestations can appear in a meaningful share of people with inflammatory bowel disease, sometimes before the digestive symptoms begin, which is one reason a new bout of shin nodules deserves evaluation [3].

How it is diagnosed

The diagnosis is mainly clinical, based on the characteristic tender nodules on the shins, with a skin biopsy reserved for atypical cases [1][2]. The more important task is looking for an underlying cause. A basic screening evaluation is worthwhile in most cases and commonly includes a complete blood count, inflammatory markers such as erythrocyte sedimentation rate or C-reactive protein, tests for recent streptococcal infection such as a throat swab and antistreptococcal antibody titers, and a chest radiograph to look for signs of sarcoidosis or tuberculosis [2]. Further tests are individualized and guided by the history and physical examination, rather than ordered indiscriminately [2]. A proposed diagnostic algorithm helps clinicians move from the initial assessment through a targeted work-up to the underlying diagnosis [1].

Treatment and what to expect

Most cases of erythema nodosum are self-limited and resolve on their own, so treatment focuses on comfort and on the underlying cause [2]. General measures include rest, leg elevation, and compression, which ease the aching [2]. Nonsteroidal anti-inflammatory drugs are commonly used first for pain when they are appropriate for the individual [2]. Because these steps relieve symptoms without addressing the trigger, identifying and treating any underlying condition is what resolves the process and lowers the chance of recurrence [1].

When the eruption is persistent or recurrent, a physician may consider additional options under specialist care, and in inflammatory bowel disease the erythema nodosum often improves as the intestinal inflammation is treated [3][4]. Any medication decision, including whether corticosteroids are appropriate, depends on excluding infection first and belongs with the treating clinician.

When to see a doctor

New, tender lumps on the shins warrant medical assessment, both to confirm the diagnosis and to check for an underlying trigger [1]. Prompt evaluation is especially sensible when the nodules come with fever, cough, breathlessness, joint pain, weight loss, or digestive symptoms, since these can point to conditions such as sarcoidosis, tuberculosis, or inflammatory bowel disease that need their own treatment [1][2][3]. Nodules that ulcerate, scar, or persist for many weeks are atypical for ordinary erythema nodosum and should be re-examined [1].

Limitations and realistic expectations

• It is usually benign but is a signal. The nodules themselves generally settle, yet they can be the first sign of a systemic condition that needs attention [1][2].
• A cause is not always found. About half of cases are idiopathic, and a normal work-up does not always yield an explanation [1][2].
• Symptom relief is not a cure of the trigger. Rest and anti-inflammatory measures ease discomfort but do not treat an underlying disease [1].
• Recurrence is possible, particularly when an underlying condition persists or a triggering exposure returns [1].
• Medication has trade-offs. Anti-inflammatory drugs and other therapies carry their own risks, and steroids are generally avoided until infection has been excluded [2].

Bottom line

Erythema nodosum is a common, usually self-limited panniculitis that presents as tender red nodules on the shins and reflects the body's reaction to a trigger, which is found in about half of cases [1][2]. Diagnosis is mainly clinical, paired with a focused search for causes such as streptococcal infection, sarcoidosis, inflammatory bowel disease, medications, and pregnancy [1][2][3]. Treatment centers on comfort and on addressing any underlying condition, and the outlook is generally good, with most cases resolving over weeks without scarring [2].

This article is for informational purposes and does not constitute medical advice.